1. Field of the Invention
The present invention relates to a surface acoustic wave device having a filtering function and favorable transmission characteristics, and to a communication apparatus including such a surface acoustic wave device.
2. Description of the Related Art
Use of a surface acoustic wave filter (hereinafter referred to as a SAW filter) having a reduced size as a filter in a mobile communication apparatus, such as a cellular phone, has been desirable as passband frequencies increase. The SAW filter includes a plurality of interdigital electrode transducers (hereinafter referred to as IDTs), each including a plurality of electrode fingers interdigitated with each other, disposed on a piezoelectric substrate along a direction of propagation of surface acoustic waves, and reflectors on both sides (left and right) of the IDTs in the direction of propagation.
Furthermore, in accordance with the recent trend for higher performance cellular phones, cellular phones including two or more communication systems to support operations in multiple bands have been developed. Furthermore, in accordance with the increase in the number of subscribers, systems that operate in a plurality of frequency bands, such as the PDC (Personal Digital Communications) 800 MHz band in Japan, have been developed.
In such cellular phones, interstage filters having large bandwidths are necessary; however, it is difficult to implement a low-loss and wide-band bandpass filter that covers two or more bands with a single SAW filter.
It is possible to use a single electronic component that functions as a wide-band bandpass filter including a plurality of SAW filters, for example, two SAW filters, that functions as a plurality of bandpass filters. In such an arrangement, either input terminals or output terminals, or both input terminals and output terminals of the two SAW filters are integrated in order to reduce the number of parts in a wireless signal section of a cellular phone.
In Japanese Unexamined Patent Application Publication No. 2001-326557 of the inventor of the present invention, published on Nov. 22, 2001, a one-input and one-output surface acoustic wave device (hereinafter referred to as a SAW device) for reception in the PDC 800 MHz band, having two passbands, in which input terminals and output terminals of two SAW filters having mutually different center frequencies are integrated, is disclosed.
As shown in FIG. 2, the SAW device includes a first SAW filter 1 and a second SAW filter 2. The first SAW filter 1 has a center frequency of 826.5 MHz to cover a frequency band ranging from 810 MHz to 843 MHz. The second SAW filter 2 has a center frequency of 877.5 MHz to cover a frequency band ranging from 870 MHz to 885 MHz. That is, the second SAW filter 2 has a higher center frequency than the first SAW filter 1.
Between an input terminal 3 and an input terminal of the first SAW filter 1, a surface acoustic wave resonator (hereinafter referred to as a SAW resonator) 5 is connected in series with the first SAW filter 1. Similarly, between the input terminal 3 and an input terminal of the second SAW filter 2, a SAW resonator 6 is connected in series with the second SAW filter 2. Furthermore, between a common node 8 between output terminals of the first and second SAW filters 1 and 2 and the output terminal of the second SAW filter 2, a SAW resonator 9 is connected in series with the second SAW filter 2.
Between a ground potential and a node 10 between the input terminal 3 and a node 7, an inductor 11 of an impedance matching circuit is connected. Similarly, between the ground potential and a node 12 between the node 8 and an output terminal 4, an inductor 13 of an impedance matching circuit is connected.
According to the arrangement disclosed in the publication described above, the passband of the SAW filter 1 having a lower center frequency in the filter characteristics is reduced by the design of the SAW filter 2.
The inventor has extensively researched the problem, and has found that the problem occurs because if the center frequencies of the two SAW filters 1 and 2 in the filter characteristics are close to each other, a spurious response caused in a lower side outside the passband of the SAW filter 2 having the higher center frequency affects the filter characteristics of the SAW filter 1 having the lower center frequency.